An asynchronous, clocked communication system with equidistant characteristics is taken to mean a system a system with the least two subscribers who are connected via a data network for the purposes of mutual exchange of data. in this case data is ex changed cyclically in equidistant communication cycles which are specified by the communication clock used by the system.
Subscribers are for example central automation devices, programming, project planning or operating devices, peripheral devices such a s input/output modules, drives, actors, sensors, Programmable Logic Controllers (PLC) or other control units, computers or machines which exchange electronic data with other machines and process data, especially from other machines. Subscribers are also called network nodes or nodes.
Control units in this document are taken to mean closed-loop controllers or control units of all types, for example coupling units (known as switches) and/or switch controllers. Typical examples of data networks used are bus systems such as Field Bus, Profibus, Ethernet, Industrial Ethernet, FireWire or also PC-internal bus systems (PCI), etc., but especially also the isochronous Realtime Ethernet.
Data networks allow communication between a number of subscribers by networking, that is connecting the individual subscribers to each other. Communication here means the transmission of data between the subscribers. The data to be transmitted is sent in this case as data telegrams, i.e. the data is packed into a number of packets and sent in this form over the data network to the corresponding recipient. The term data packet is thus used. The term transmission of data is used in this document fully synonymously with the above-mentioned transmission of data telegrams or data packets.
In distributed automation systems, for example in the area of drive technology, specific data must arrive at specific times at the intended subscribers and must be processed by the recipients. This is referred to as realtime-critical data or data traffic since if the data does not arrive at its intended destination at the right time this can produce undesired results at the subscriber by contrast with non-realtime critical, for example Internet or Intranet based data communication. In accordance with the IEC 61491, EN61491 SERCOS interface successful realtime critical data traffic of the type mentioned can be guaranteed in distributed automation systems.
Automation components (e.g. controllers, drives, . . . ) nowadays generally have an interface to a cyclically clocked communication system. A run level of the automation components (fast-cycle) (e.g. positional control in a controller, torque control of the drive) is synchronized to the communication cycle. This defines the communication timing. Other lower-performance algorithms (slow-cycle) (e.g. temperature controllers) of the automation components can also only communicate via this communication clock with other components (e.g. binary switches for fans, pumps, . . . ), although a slower cycle would be adequate. Using only one communication clock for transmission of all information in the system produces high demands on the bandwidth of the transmission link.
A peripheral image is made up of a sum of data sets which are exchanged using realtime communication with other automation devices. Data sets which are received from an automation device using realtime communication are input data. Data sets which are sent from an automation device using realtime communication are output data. In an automation device input data is processed and new output data created in an application program which is called in cycles. The application program can comprise a number of different functions which work at different times with different data sets. It is not mandatory for all functions of the application program to be called in each application cycle. This means that all input data is not processed and new output data generated in each application cycle.
A system and a method for transmitting realtime critical and non-realtime critical data via switchable data networks is known from DE 10058524 A1. Here the transmission cycle is subdivided into a subcycle for the realtime critical data and a subcycle for the non-realtime critical data.
A common disadvantage of communication systems with realtime capabilities known from the prior art is that existing non-realtime capable system components cannot be inserted into the real time capable communications system. For this reason setting up a realtime capable communication system requires a high level of investment since all the existing non-realtime capable systems have to be replaced.